In general, group-III nitride semiconductors (devices) have a direct bandgap structure and a tunable bandwidth between approximately 0.7 and 6.2 electron-Volts (eV). These characteristics make group-III nitride semiconductors attractive for use in various applications in electronics and optoelectronics. However, heteroexpitaxial grown group-III nitride films lack a native crystal nitride, which results in a large lattice mismatch between nitride films and foreign substrates. Additionally, a large interface energy that is present due to the large lattice mismatch makes epitaxial nucleation on the foreign substrate surface difficult (“non-wetting problem”). As a result, the materials grown can be randomly oriented causing a relatively rough surface and, in the worst case, the materials can grow as a polycrystalline material. These shortcomings cause a relatively large number of defects and limit the ability to realize use of group-III nitride semiconductors in most state-of-the-art applications.
Various approaches seek to address epitaxial nucleation on the foreign substrate. In one approach, sapphire is used as the substrate and a low-temperature grown Aluminum Nitride (AlN) or Gallium Nitride (GaN) buffer layer is grown. In particular, the sapphire substrate is heated to 1000-1200 degrees Centigrade (C) to remove surface contamination. The temperature is then reduced to 400-1000 degrees C. and a metalorganic material and a nitrogen source are simultaneously applied to the substrate to form the low-temperature buffer layer. The supply of metalorganic material is then stopped and the temperature is raised to crystallize the low-temperature buffer layer. Subsequently, a desired group-III nitride semiconductor crystal can be epitaxially grown on the buffer layer.
Since low-temperature grown AlN and GaN are amorphous, the buffer layer will not suffer from the non-wetting problem. Additionally, the buffer will provide the desired epitaxial growth for the later high-temperature nitrides growth. However, the low-temperature buffer has a very poor crystalline quality. This results in a large number of defects, many of which extend to the top layers and become a major killing factor for the state-of-the-art nitride devices.
In another approach, group-III metal particles are pre-deposited, nitridated, and used as nucleation sites for following growths. In particular, a sapphire substrate is heated up to 1180 degrees C. in a hydrogen atmosphere and held at that temperature for ten minutes to remove any oxide film from the substrate surface. Subsequently, the temperature is reduced to 1100 degrees C., and in the same hydrogen atmosphere without a nitrogen source, a metalorganic material, trimethyl aluminum (TMA), is supplied to the substrate for one minute at a flow rate of twelve μmol/min. The TMA is thereby thermally decomposed, resulting in the deposition of Al particles on the sapphire substrate. After shutting off the TMA, the temperature is raised to 1180 degrees C. and a nitrogen source, in the form of ammonia (NH3), is supplied for three minutes at a flow rate of 0.2 mol/min, nitridating the Al. Next, with the NH3 flow rate unchanged and the temperature maintained at 1180 degrees C., a metalorganic material, trimethyl gallium (TMG), is supplied at a flow rate of 140 μmol/min causing epitaxial growth of GaN on the substrate with the Al deposition. However, even this approach yields large densities of screw dislocations and other defects.
In view of the foregoing, a need exists to overcome one or more of the deficiencies in the related art.